1. Field of the Invention
The present invention relates to a blood-pressure monitoring apparatus which monitors a blood pressure of a living subject based on information which changes in relation to the change of blood pressure of the subject.
2. Related Art Statement
As information relating to a pulse wave which propagates through an arterial vessel of a living subject, there are known a pulse-wave-propagation time DT and a pulse-wave-propagation velocity V.sub.M (m/s). The pulse-wave-propagation time DT is a time which is needed by the pulse wave to propagate between two different positions of the arterial vessel. Additionally, it is known that the above pulse-wave-propagation-relating information is, within a predetermined range, substantially proportional to the blood pressure ("BP", mmHg) of the living subject. Hence, there has been proposed a BP monitoring apparatus which determines, in advance, coefficients .alpha., .beta. in the following expression: EBP=.alpha.(DT)+.beta. (where a is a negative constant and .beta. is a positive constant), or EBP=.alpha.(V.sub.M)+.beta. (where .alpha. and .beta. are positive constants), based on two measured BP values of the subject and two measured pulse-wave-propagation time values (DT) or two measured pulse-wave-propagation velocity values (V.sub.M), iteratively determines an estimated BP value EBP of the subject, based on each piece of subsequently obtained pulse-wave-propagation-relating information, according to the above-indicated first or second expression, and starts a BP measurement using an inflatable cuff when an estimated BP value EBP has changed by not less than a predetermined amount or proportion from a prior value EBP determined at the time of the last BP measuring operation.
A relationship between pulse-wave-propagation-relating information and estimated BP value BP, represented by the above-indicated first or second expression, changes depending upon the condition of a central organ of the living subject (e.g., the condition of cardiac muscle) and/or the condition of a peripheral organ of the subject (e.g., the hardness of peripheral blood vessels, or the resistance of those vessels to the blood flow). Thus, there has been proposed a BP monitoring apparatus which obtains hertbeat-relating information (e.g., heart rate or pulse period) as central-organ-relating information, and peripheral-pulse-wave-area-relating information as peripheral-organ-relating information and which starts a BP measurement using an inflatable cuff when the pulse-wave-propagation-relating information (or the estimated BP value EBP calculated based on the pulse-wave-propagation-relating information) has changed by not less than a predetermined amount or proportion from the time of the last BP measuring operation and simultaneously when at least one of the hertbeat-relating information and the peripheral-pulse-wave-area-relating information has changed by not less than a predetermined amount or proportion from the time of the last BP measuring operation. An example of this BP monitoring apparatus is disclosed in U.S. Pat. No. 5,752,920.
However, the prior BP monitoring apparatuses as described above display only a digital value indicative of each piece of BP-relating information which is used in judging whether or not to start a BP measurement using the cuff, or a trend graph representing a timewise change of a plurality of pieces of BP-relating information. From only those items, however, the subject or a medical person who attends to the subject cannot judge whether the BP of the subject is near to, or is changing toward, a state in which a BP measurement using the cuff is started. Accordingly, the medical person can not give any treatments to the subject before a BP measurement using the cuff is actually started.